We have previously shown that a sequence more than 200 base pairs long is involved in the specific recognition of the phage attachment site of bacteriophage Lambda. We have also shown that a DNA molecule carrying the 15 base pair long common core sequence retained the capacity to act as the bacterial attachment site (att B), but poorly. Those molecules that carried two additional four base pair sequences of the original DNA on both sides of the common core were as active as the authentic att B. The nature of the cross-over point within the common core sequence during the intergrative recombination reaction has been investigated. The disintegration of 32P atoms incorporated in a DNA molecule was used to generate base specific cleavage within a segment of DNA that derived from one of the parents of the reaction. Integrative recombination was shown to involve staggered cuts at unique sites within the common core sequence. Under collaboration with Howard Nash, we found that the integrase, that was shown to have DNA topoisomerase activity, is a type I DNA Topoisomerase.